13.5: Composition of Body Fluids

The components of the ECF — plasma and IF — bear greater resemblance to each other compared to the ICF (Table 3). Plasma contains elevated concentrations of sodium, chloride, bicarbonate ions and proteins. The composition of these solutes are similar in the IF, except for significantly lower protein levels. On the other hand, the ICF demonstrates higher levels of potassium, phosphate, and magnesium ions, with notably increased protein concentrations compared to the ECF.

Body fluids generally maintain a neutral charge. This equilibrium in fluids involves a balance of cations and anions. Although sodium and potassium ions can move through leak channels with sodium ions entering cells and potassium ions leaving cells, the high levels of potassium ions and low levels of sodium ions within the ICF are upheld by sodium-potassium exchange pumps located in cell membranes.⁵

​Table 3 | The Concentrations of Electrolytes in the Body Fluids |

Electrolytes	Plasma (mEq/L)	Interstitial fluid (mEq/L)	Intracellular fluid (mEq/L)
Sodium	142	145.1	12.0
Potassium	4.3	4.4	150
Chloride	104	117.4	4.0
Bicarbonate	24.0	27.4	12.0

 

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